Heat transfer element

ABSTRACT

An elongate tubular heat transfer element having a longitudinal tube axis that runs through the hollow interior of the heat transfer element. The heat transfer element includes a wall of monolithic construction having an outer surface and an inner surface. The wall is formed from a composite material including a matrix and rovings embedded in the matrix. The composite material is in contact with the hollow interior such that the inner surface determines a boundary of the hollow interior which extends longitudinally along the axis of the heat transfer element. The matrix is of a fluoropolymer having embedded therein rovings of boron-free chemically resistant glass fibres. The rovings include from about 20% to about 60% by volume based upon the volume of the composite material, rovings that extend longitudinally in a lengthwise direction parallel to the axis of the heat transfer element and rovings that extend spirally around the axis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat transfer element, moreparticularly to a heat transfer element for use in a power generatingstation or a chemical processing plant. Such a heat transfer element canbe in the form of a sheet or tube, for example.

2. Background

There are currently over six hundred power generating stations in theEuropean Union. An important feature of these stations is the provisionof heat exchangers consisting of a number of radiant panels which serveto transfer heat within the station. There may be around 30,000 squaremeters of radiant panels in a single heat exchanger. A power generatingstation may use up to twelve or more heat exchangers.

The radiant panels should not only serve their primary heat transferfunction, they should also be robust to withstand the conditions inwhich they operate. Thus, not only are physical conditions harsh, withhot air and steam at up to about 150° C. flowing at high speed past thepanels, but also corrosive chemicals, such as sulphurous and nitrousacids, are present in the air stream. Furthermore, the panels may becomeclogged with soot or debris, which may also impair their function. Thepanels are also subjected to rapid thermal cycling.

Conventionally, heat transfer elements used to make the radiant panelshave been manufactured from a metal with a vitreous enamel coating. Themetal base material, conveniently of mild steel, provides the necessarystructural strength to the element and also the required thermalconductivity. A coating of vitreous enamel protects the metal base fromthe corrosive effects of the surrounding environment.

Recently, attempts have been made to provide heat transfer elements byspraying a metal base with a fluoropolymer. However, the resultingcomposite element is not economical to manufacture.

In U.S. Pat. No. 4,461,347 there is proposed a heat exchanger assemblycomprising coaxially arranged inner and outer pipes. The inner pipe canbe formed of high strength metal and ensheathed by an extruded heatshrinkable plastics tube of non-reactive material, such aspolytetrafluoroethylene or polypropylene.

A plate heat exchanger comprising at least three plate elementsconsisting of graphite and a fluoropolymer, such as polyvinylidenefluoride is disclosed in European Patent Specification No. 0 203 213 A1.

British Patent Specification No. 2 255 148A teaches a structurallycomposite metal and plastics tube in which the metal forms a tubularcore having openings throughout its length occupying at least 5% of itstotal surface area while the plastics material forms imperforate innerand outer layers, each at least 0.1 mm thick, covering the inside andoutside of the metal core and integrally joined through the openings.

There is a need to improve upon the performance of heat transferelements in power generating stations. Thus, it would be desirable toprovide a heat transfer element with improved heat transfer properties,with improved anti-fouling properties, with improved resistance tophysical and chemical corrosion, and with improved mechanicalproperties.

All of these desiderata are objects of the present invention.

A further object of the present invention is to provide a heat transferelement with the improved properties referred to above but which isrelatively economical to manufacture.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided aheat transfer element comprising a polymer matrix having a fibrousmaterial interspersed therein, said heat transfer element comprising afluoropolymer at least on an outer surface thereof, the interspersion ofthe fibrous material within the polymer matrix providing rigidity to theheat transfer element, a thermally conductive material being distributedwithin the heat transfer element.

The fibrous material may comprise metal fibres, such as iron, steel, orstainless steel fibres, in which case additional thermally conductivematerial is not necessary. However, it is also possible, when usingmetal fibres, to add a particulate metal such as particles of iron,steel, stainless steel or copper.

The fibrous material may alternatively comprise glass fibres, preferablyglass fibres made from a chemically resistant glass, for exampleboron-free glass fibres, or a mixture of glass fibres and fibres of aplastics material, such as polypropylene or a fluoropolymer.

It is also contemplated that the fibrous material can comprise glassfibres coated with a thermally conductive material.

The fibrous material can be incorporated in any convenient form.Preferably the fibrous material comprises continuous fibres in one ofthe forms conventionally used for making fibre reinforced articles.Examples include randomly distributed or closely mingled fibres, orrovings braided to form continuous tubes, formed into preimpregnatedtapes, or woven into panels. The rovings may themselves be precoatedwith, for example, a plastics material. One form of continuous tubecomprises loosely commingled or interwoven rovings, for example looselyinterwoven glass fibre rovings, wherein the individual rovings extend ata small angle, for example about 10° to about 15°, to the tube axis.Such glass fibres may be intermingled with polypropylene fibres or withfluoropolymer fibres or coated with polypropylene powder orpolyvinylidene powder. Another form of fibrous material which can beused in the practice of the invention comprises a narrow band ofparallel fibres as warp interwoven with a similar narrow band ofparallel fibres as weft, with the warp and weft crossing each othersubstantially at right angles to one another. Such narrow bands may be,for example, from about 0.2 cm to about 2 cm wide.

It is also possible to use a mixture of metal and glass fibres as thefibrous material.

Thus one preferred from of heat transfer element according to theinvention comprises:

a polymer sheet having a fibrous material interspersed therein andcomprising a fluoropolymer at least on an outer surface of the sheet,the interspersion of the fibrous material within the sheet providingrigidity to the element; and

a thermally conductive material distributed within the heat transferelement.

Heat transfer elements according to the invention have a number ofsignificant advantages over conventional heat transfer elements, inparticular the conventional elements used to form the radiant panels ofpower generating stations.

The provision of a fluoropolymer sheet significantly improves theanti-fouling properties of the heat transfer elements of the invention.Fluoropolymers have low surface energy and good lubricity and aretherefore able to resist fouling by soot and debris to a greater extentthan has been the case with conventional ceramic materials. Furthermore,fluoropolymers tend to be extremely resistant to chemical attack and arewell adapted to withstand the corrosive action of the sulphurous andnitrous acids present in the air stream flowing past the elements whenin use. This resistance to chemical attack prevents surface solvation,which could otherwise worsen the flow characteristics of the surface.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the invention, the fibrous material is itself athermally conductive material, for example a metal such as iron, mildsteel, or stainless steel.

One advantage of using a thermally conductive material as the fibrousmaterial is that it may not then be necessary to provide any furtherthermally conductive material in the element. In this case, the fibrousmaterial will itself serve as the sole thermally conductive material inthe element. However, it may in some cases be preferred to distribute athermally conductive material within the element by means other than thefibrous material. Thus, in one preferred embodiment of the invention,the thermally conductive material comprises a particulate or filamentedmaterial, for example, a particulate or filamented metal such as iron orsteel. This particulate or filamented material may be mixed with thefluoropolymer prior to compression moulding or lamination of thefluoropolymer onto the fibrous material. The resulting heat transferelement according to the invention will comprise a fibrous material,which may if desired be of metal or some other thermally conductivematerial but which may alternatively be or include a thermal insulatoror a material having a relatively low thermal conductivity, such asglass fibres, preferably made from chemically resistant glass such asboron-free glass, and a fluoropolymer sheet having the thermallyconductive particulate or filamented material distributed within thefluoropolymer sheet or polymer matrix.

Although glass fibres exhibit relatively low thermal conductivityproperties, it has been found that adequate thermal conductivity can beimparted to the heat transfer elements of the invention by utilisinghigh volume proportions of glass fibres, for example up to about 60% byvolume of the heat transfer element. The use of such levels of glassfibres is economically advantageous because the polyvinylidene fluorideor other fluoropolymer is typically about 6 times more expensive thanglass fibres. Hence the invention enables the production of heattransfer elements in a relatively economical manner, even thoughutilising a relatively expensive fluoropolymer in its manufacture.

In general the desired heat conductivity properties can be achieved byvarying the loading of the fibrous material and/or by mixing a fillerwith good thermal conductivity properties such as metal fibres or metalpowder with a material with lower thermal conductivity such as glassfibres. Typically the amount of glass fibres can range from about 20% byvolume to about 60% by volume of the heat transfer element. Theproportion of metal fibres or particles used can range up to about 25%by volume but is usually not greater than about 20% by volume of theheat transfer element.

The polymer sheet or matrix may consist entirely of a fluoropolymer oradmixtures of a fluoropolymer with compatible thermoplastic polymers,antioxidants and other additives. In this case the fibrous material isinterspersed within the fluoropolymer. This can be achieved bylaminating a pad of fibrous material, for example a pad of chemicallyresistant glass fibres or metal fibres, between two sheets or films offluoropolymer. However, in an alternative embodiment of the invention,the polymer sheet may comprise an underlayer of a plastics material, inwhich the fibrous material is interspersed, and an overlayer offluoropolymer. The plastics material is preferably an acrylic polymer oralloy. This arrangement may be desirable for economic reasons. When theplastics material, such as a relatively inexpensive acrylic polymer, islaminated or compression moulded onto the fibrous material, thethermoplastic acrylic polymer flows into and around the fibres andprovides a relatively cheap filler onto which the fluoropolymer may becoated. Of course, the lamination or compression moulding of the fibrousmaterial with the inexpensive acrylic filler and the fluoropolymer maybe done simultaneously by applying heat and pressure to a sandwichhaving an outer film of fluoropolymer, an intermediate layer of acrylicpolymer and an inner layer of fibrous material. In this case, thefibrous material may become interspersed in both the acrylic polymer andthe fluoropolymer.

The use of compression moulding or lamination, for example continuousbelt lamination, to form the heat transfer element is preferred,particularly when forming the heat transfer element as a sheet. However,it may sometimes be appropriate, for example when an inexpensive acrylicpolymer is used, to powder coat the fluoropolymer onto a base portionformed after cooling of the acrylic base sheet with interspersed fibrousmaterial. However, the use of compression moulding or lamination allowsthe manufacturer to minimise the thickness of the coating, thusimproving the thermal transfer properties of the element and allowingcost-effective manufacture of the element by minimising the quantity ofthe expensive fluoropolymer used therein.

Typically a heat transfer element in the form of a sheet has an overallthickness of from about 0.4 mm to about 1.2 mm.

The heat transfer element of the invention may also be formed as a tubeby extrusion of a fluoropolymer melt and interspersed fibrous material.Other conventional methods of forming fibre reinforced plastics tubesmay be used. For example, a tube can be formed by spirally winding oneor more layers of a fibre reinforced plastics tape on to a mandrel andcompressing or fusing the tape portions one to another as appropriate.If more than one layer of tape is used then the fibre directions of thetwo layers can be different. If the tape does not itself comprise afluoropolymer, then a fluoropolymer tape or film can simultaneously orthereafter be applied to the fibre reinforced layer or layers andlaminated thereto by application of heat and/or pressure. If the fibrousreinforcement is a poor conductor, for example glass fibres, then metalpowder or metal fibres can be incorporated either in the fibrereinforced layer or in the fluoropolymer coating layer.

Suitable equipment for manufacture of tubular heat transfer elements inaccordance with the invention can be achieved using, for exampletechnology developed by Automated Dynamics of 407 Front Street,Schenectady, N.Y. 12305, United States of America in order to effectfibre placement during tube formation, or the discontinuous doublepressing operation as provided by BST Beratung und System Technik GmbHof Am Flughaven 7613, 88406 Friedrichshafen, Germany.

The tube or pipe can be of any convenient cross section such as round,oval or square. It can have fins or other structural features integrallyformed therewith. Its diameter can vary within wide limits, for examplefrom about 1 cm up to about 25 cm or more, e.g. about 38 mm. It can havecouplings or other fittings integrally moulded therein. The tube or pipecan vary in internal dimensions or wall thickness along its length.

When the heat transfer element of the invention comprises a sheet, itcan be bent, corrugated or otherwise formed into a desired shape, usingappropriate conditions of heat and/or pressure.

The fluoropolymer used in the present invention is preferably afluorohydrocarbon polymer, such as polyvinylidene fluoride (PVDF) or acopolymer with at least 80% by weight of vinylidene fluoride and up to20% by weight of at least one other fluorine based monomer. Suitablefluorine based monomers which may be used with vinylidene fluoride aretetrafluoroethylene, hexafluoropropylene and vinyl fluoride, having thecharacteristics listed in U.S. Pat. Nos. 4,770,939 and 5,030,394. Thefluoropolymer is most preferably PVDF and is commercially available fromAtochem North America, Inc. under the trade designation KYNAR 500 PC,KYNAR 710, KYNAR 711 or KYNAR 2800.

The fluoropolymer may be mixed with another thermoplastic polymer. Thepreferred thermoplastic polymers are acrylic polymers with units derivedfrom acrylates or methacrylates, such as copolymers derived from analkyl acrylate or alkyl methacrylate, preferably, methyl methacrylate orfrom at least one other olefinically unsaturated monomer. Acrylic acidand methacrylic acid are also suitable as the other olefinicallyunsaturated monomer. Advantageously, the copolymers comprise at least75% by weight of units derivable from an alkyl methacrylate and up to25% by weight of units derivable from one or more other olefinicallyunsaturated monomers. The thermoplastic polymer is preferablypoly(methyl acrylate) or poly (methyl methacrylate) or an alkylmethacrylate/alkyl acrylate copolymer. These thermoplastic polymers havethe characteristics listed in U.S. Pat. Nos. 4,770,939 and 5,030,394 andare commercially available from Rohm & Haas Company under the tradedescription Acryloid/Paraloid B-44®. These materials are described inU.S. Pat. No. 5,229,460. Another preferred acrylic polymer is availablefrom Atohaas under the trade designation OROGLAS HFI-10.

The use of an acrylic polymer in admixture with the fluoropolymer canimprove the wetting properties of the material and thus help to ensureeven coating of the fibrous material in the heat exchange element of theinvention.

The weight ratio of the fluoropolymer to the thermoplastic acrylicpolymer, if used, is preferably in the range of from about 90:10 to40:60, preferably from about 75:25 to 65:35, for example about 70:30.

A low melting point fluorine-based terpolymer may also be added to thefluoropolymer/thermoplastic acrylic polymer mixture. A terpolymer is apolymer made from three monomers. Such a low melting point terpolymerwould have, for example, a melting point of not higher than 150° C. Asuitable terpolymer is vinylidenefluoride-tetrafluoroethylene-hexafluoropropylene, having a meltingtemperature of about 870 to 93° C. and a melt viscosity of about 11,000to 13,000 Poise at 125° C. The preferred terpolymer is commerciallyavailable from Atochem, North America, Inc. under the trade designationKYNAR ADS®. The weight ratio of the fluoropolymer to the terpolymer, ifused, is in the range of from about 50:50 to 99:1.

The mixture may also contain other additives, such as corrosioninhibiting pigments, dry flow promoting agents, antioxidants, adhesionpromoters and ultra-violet-absorbing materials, although not required.One preferred additive is an antioxidant, such as2,2-bis[3-[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]-1-oxopropoxy]methyl]-1,3-propanediyl3,5-bis(1,1-dimethylethyl)-4-hydroxybenzene-propanoate, available fromCiba-Geigy under the trade designation Irganox 1010.

The fluoropolymer composition can be formed into a thin film forlamination to the outside of a heat transfer element in accordance withthe invention.

In order that the invention may be properly understood and fully carriedinto effect, a number of preferred embodiments thereof will now be moreparticularly described in the following Examples:

EXAMPLE 1

A fluoropolymer composition comprising the following ingredients wasprepared:

Raw Materials % by weight Kynar ™ 710 69.3% Paraloid ™ B-44 Beads 29.7%Irganox ™ 1010 1.0%

The materials were mixed in a high speed MIXACO™ mixer and fed into atwin screw extruder and extruded at about 200° C. The extrudate wasquenched in a water bath and then pelletised.

The pelleted composition was extruded through a single screw extruderwith a single slot die to form a continuous film with a thickness ofaround 120 μm.

The resulting film was used to coat a fibrous pad of mild steel byplacing a sheet of film on each side of the pad and subjecting thecovered pad to a temperature of 200° C. and =a pressure of 0.625 tonnesper square inch (95 bar) in a heating press.

The resulting heat transfer element has a thickness of about 1 mm andhas excellent heat transfer, anti-fouling, structural and flowcharacteristics.

EXAMPLE 2

A film of fluoropolymer coating composition was prepared as describedabove in Example 1 and was used to coat a fibrous mild steel pad bycovering both sides of the pad with film and passing the covered padthrough a twin belt laminator. Acetate release sheets were placed overthe fluoropolymer film to prevent adherence of the fluoropolymer to thebelts of the laminator.

The resulting heat transfer element is approximately 1 mm thick and hasexcellent heat transfer, anti-fouling, structural and flowcharacteristics.

EXAMPLE 3

A fluoropolymer coating composition as specified in Example 1 wasprepared and mixed with stainless steel filings in a ratio of threeparts by weight of the coating composition to one part by weight ofstainless steel filings. The resulting composite material was laminatedonto a fibre glass pad using the method described in Example 2 to form aheat transfer element having a thickness of about 1 mm with excellentheat transfer, anti-fouling, structural and flow characteristics.

EXAMPLE 4

Examples 1 to 3 were repeated using a fluoropolymer composition of thefollowing ingredients:

Raw Materials % by weight Kynar ™ 2800 60.00% Oroglas ™ HFI-10 40.00%

In each case, a heat transfer element with excellent heat transfer,anti-fouling, structural and flow characteristics was produced.

EXAMPLE 5

A laminate comprising two pre-manufactured Solex 8008 100% fluoropolymerfilms, each 0.150 mm thick, and two 110 g/m² Advantex™ pre-manufacturedfibrous chemically resistant glass mats were combined together with afibrous pad of steel approximately 0.6 mm thick by laminating themtogether in a twin belt laminator using a pressure of less than 5 barand a temperature of 230° C. The resulting laminate has a thickness of0.91 mm and has excellent economic performance, and heat transfer,anti-fouling, structural and flow characteristics.

EXAMPLE 6

A pipe is prepared by tape winding preprepared tapes comprising 60% byvolume chemically resistant glass fibre together with 40% by volume ofKynar 711. This was obtained in the form of a very fine powder and wascoated using a fluidised bed on to the glass fibres and thenconsolidated using a heated die. The resultant tape was 0.4 mm thick and20 mm wide and was wound on to a mandrel with 60% of the tape in thelength of the pipe and 40% in the inner and outer surfaces of the pipeat an angle of +/−20°. The resultant pipe performed well under test.

1. An elongate tubular heat transfer element having a longitudinal tubeaxis that runs through the hollow interior of the tubular heat transferelement wherein the tubular heat transfer element comprises a wall ofmonolithic construction having an outer surface and an inner surface,said wall being formed from a composite material comprising a matrix androvings embedded in the matrix, wherein the composite material is incontact with the hollow interior, such that the inner surface determinesa boundary of the hollow interior which extends longitudinally along thetube axis of the heat transfer element and wherein the matrix consistsessentially of a fluoropolymer selected from polyvinylidene fluoride andcopolymers of at least 80% by weight, based upon the weight of thecopolymer, of vinylidene fluoride and up to 20% by weight, based uponthe weight of the copolymer, of at least one other fluorine basedmonomer selected from tetrafluoroethylene, hexafluoropropylene and vinylfluoride, and wherein the rovings embedded in the matrix compriseboron-free chemically resistant glass fibres, the rovings comprisingfrom about 20% to about 60% by volume based upon the volume of thecomposite material and including rovings which extend longitudinally ina lengthwise direction parallel to the tube axis of the tubular heattransfer element and rovings which extend spirally around the tube axis.2. An elongate tubular heat transfer element according to claim 1,wherein the fluoropolymer is polyvinylidene fluoride.
 3. An elongatetubular heat transfer element according to claim 1, further comprising afirst layer adjacent the outer surface of the wall, a second layersurrounding the first layer, and at least one other layer intermediatethe first and second layers, wherein the first, second and at least oneother layers each include a plastics material and rovings embedded inthe plastics material, and wherein the rovings of a particular layer allextend substantially in a common direction which is different from thecommon direction of any adjacent layer, and wherein the common directionis in each case selected from a direction extending spirally around thetube axis and a direction extending longitudinally in a lengthwisedirection parallel to the tube axis.
 4. An elongate tubular heattransfer element according to claim 1, wherein the wall comprises afirst layer adjacent the inner surface, a second layer adjacent theouter surface, and an intermediate layer between said first and secondlayers, wherein the first, second and intermediate layers each include aplastics material and rovings embedded in the plastics material andwherein the rovings in the first layer of the wall adjacent the innersurface and the rovings in the second layer of the wall adjacent theouter surface each extend spirally around the tube axis and wherein therovings in the intermediate layer of the wall between the first andsecond layers extend longitudinally in a lengthwise direction relativeto the tube axis of the tubular heat transfer element.
 5. An elongatetubular heat transfer element according to claim 4, wherein the rovingsin the intermediate layer comprise about 60% of the total rovings andwherein the rovings of the first and second layers together compriseabout 40% of the total of all rovings in the heat transfer element. 6.An elongate tubular heat transfer element according to claim 1, whereinthe composite material further comprises a particulate metal.
 7. Anelongate tubular heat transfer element according to claim 1, wherein thecomposite material further comprises a particulate thermally conductivematerial.
 8. An elongate tubular heat transfer element having alongitudinal tube axis that runs through the hollow interior of thetubular heat transfer element wherein the tubular heat transfer elementcomprises a wall of monolithic construction having an outer surface andan inner surface, said wall being formed from a composite materialcomprising a matrix and rovings embedded in the matrix, wherein thecomposite material is in contact with the hollow interior, such that theinner surface determines a boundary of the hollow interior which extendslongitudinally along the tube axis of the heat transfer element andwherein the matrix consists essentially of polyvinylidene fluoridehaving embedded therein rovings of boron-free chemically resistant glassfibres, the rovings comprising from about 20% to about 60% by volumebased upon the volume of the composite material and including rovingswhich extend longitudinally in a lengthwise direction parallel to thetube axis of the heat transfer element and rovings which extend spirallyaround the tube axis.
 9. An elongate tubular heat transfer elementaccording to claim 8, further comprising a first layer adjacent theouter surface of the wall, a second layer surrounding the first layer,and at least one other layer intermediate the first and second layers,wherein the first, second and at least one other layers each include aplastics material and rovings embedded in the plastics material, andwherein the rovings of a particular layer all extend substantially in acommon direction which is different from the common direction of anyadjacent layer, and wherein the common direction is in each caseselected from a direction extending spirally around the tube axis and adirection extending longitudinally in a lengthwise direction parallel tothe tube axis.
 10. An elongate tubular heat transfer element accordingto claim 8, wherein the wall comprises a first layer adjacent the innersurface, a second layer adjacent the outer surface, and an intermediatelayer between said first and second layers, wherein the first, secondand intermediate layers each include a plastics material and rovingsembedded in the plastics material and wherein the rovings in the firstlayer of the wall adjacent the inner surface and the rovings in thesecond layer of the wall adjacent the outer surface each extend spirallyaround the tube axis and wherein the rovings in the intermediate layerof the wall between the first and second layers extend longitudinally ina lengthwise direction relative to the tube axis of the tubular heattransfer element.
 11. An elongate tubular heat transfer elementaccording to claim 10, wherein the rovings in the intermediate layercomprise about 60% of the total rovings and wherein the rovings of thefirst and second layers together comprise about 40% of the total of allrovings in the heat transfer element.
 12. An elongate tubular heattransfer element according to claim 8, wherein the composite materialfurther comprises a particulate metal.
 13. An elongate tubular heattransfer element according to claim 8, wherein the composite materialfurther comprises a particulate thermally conductive material.